题目链接：http://acm.pku.edu.cn/JudgeOnline/problem?id=3659

这题不算难题了，基本算是中等题

题目大意是给出一颗树，在一些点建一个信号塔，信号塔覆盖范围是其所在点和邻近点，问最少几个信号塔可以覆盖全区域

思路是树状DP（后序遍历），有三个状态，分别记录父节点建塔，本节点建塔和子节点建塔的最少建塔数量

需要注意的就是子节点建塔的判断，子节点建塔只要一个以上的子节点建塔即可

代码如下：

#include <iostream>
#include <cstdio>
#include <cstring>
#include <vector>
#include <cmath>
using namespace std;

struct node
{
    vector<int> linkto;
    int build[3];//0 为自己建塔， 1 为父节点建塔， 2为子节点建塔
};

node pasture[10005];

int calcPos(int pos, int f);
int main()
{
    int i, j, n, a, b, res;

    scanf("%d", &n);
    for(i = 0; i < n - 1; i ++)
    {
        scanf("%d %d", &a, &b);
        pasture[a - 1].linkto.push_back(b - 1);
        pasture[b - 1].linkto.push_back(a - 1);
    }

    calcPos(0, 0);
    res = min(pasture[0].build[0], pasture[0].build[2]);

    printf("%d\n", res);
    return 0;
}

int calcPos(int pos, int f)
{
    int i, tmp;
    bool flag = false;
    pasture[pos].build[0] = 1;
    pasture[pos].build[1] = 0;
    pasture[pos].build[2] = 0;

    for(i = 0; i < pasture[pos].linkto.size(); i ++)
    {
        if(pasture[pos].linkto[i] != f)
        {
            calcPos(pasture[pos].linkto[i], pos);
            pasture[pos].build[0] += min(min(pasture[ pasture[pos].linkto[i] ].build[0], pasture[ pasture[pos].linkto[i] ].build[1])
                , pasture[ pasture[pos].linkto[i] ].build[2]);
            pasture[pos].build[1] += min(pasture[ pasture[pos].linkto[i] ].build[0], pasture[ pasture[pos].linkto[i] ].build[2]);
            pasture[pos].build[2] += min(pasture[ pasture[pos].linkto[i] ].build[0], pasture[ pasture[pos].linkto[i] ].build[2]);
            flag = flag || (pasture[ pasture[pos].linkto[i] ].build[0] <= pasture[ pasture[pos].linkto[i] ].build[2]);
        }
    }

    if(pasture[pos].build[2] == 0)
    {
        pasture[pos].build[2] = 100000;
        return 0;
    }
    if(flag == false)
    {
        tmp = 100000;
        for(i = 0; i < pasture[pos].linkto.size(); i ++)
        {
            if(pasture[pos].linkto[i] != f
                && pasture[pasture[pos].linkto[i]].build[0] - pasture[pasture[pos].linkto[i]].build[2] < tmp)
            tmp = pasture[pasture[pos].linkto[i]].build[0] - pasture[pasture[pos].linkto[i]].build[2];
        }
        pasture[pos].build[2] += tmp;
    }
    return 0;
}